1. Field of the Invention
The invention relates to a technical pen configured particularly for use in drafting plotters, with a writing tube provided in the front end into which a cleaning wire extends from the rear. The cleaning wire is fixed at the front end of a drop weight body which is movable back and forth axially to a limited degree in an inner bore. The inner bore is connected with a writing fluid reservoir, that is coaxial to the inner bore and a pressure equalization chamber. The inner end of the pressure equalization chamber is connected with the writing fluid reservoir and the outer end of the pressure equalization chamber is connected with the ambient air.
2. Brief Description of the Prior Art
Technical pens are known to have various shapes. A pressure equalization chamber is customarily provided in a wall surrounding the inner bore. The inner end of the pressure equalization chamber generally is connected via the inner bore with the writing fluid reservoir. The outer end of the pressure equalization chamber is located in an area of the front end of the inner bore that is close to the back end of the writing tube and is open towards the front. The writing fluid reservoir is formed by a cartridge or the like, and is disposed behind the inner bore. In this way the writing fluid generally enters the inner bore axially through an opening provided on the back end of the inner bore.
When a technical pen of the known type is used as a hand-held instrument, the writing fluid entering the pressure equalization chamber may possibly dry out in the chamber. This may lead to soiling of the pressure equalization chamber, which may even result in adjoining parts sticking to each other so that they are very difficult to separate.
When a technical pen is used as a so-called plotter pen, i.e. as the writing element in the plotter head of a plotter, this difficulty occurs in an even more pronounced way, because the front end of the writing tube of the technical pen is placed on the drafting surface with a bump when the plotter head is lowered. The bumps result in the writing fluid in the area of the pressure equalization chamber being tossed forward. Hence, the danger of writing fluid drying out and clogging the equalization chamber is increased.